A big problem with adding in playability for unlanded characters (and barons, for that matter) is that unlanded characters (and, to some extent, barons) aren't really AI-controlled. Anything going on with them is purely event-driven. If they were to be played, they would have to be able to receive events/take decisions in much the same way as a landed ruler, while at the same time not being able to take decisions/receive events that only should work for landed characters (which would not cause a slowdown in itself but might require some serious recoding of some events depending on the existing checks). If that was the case, the slowdown would be terrible, making the current endgame lag feel mild in comparison.
There's something like 1000 counties in the game, most with several baronies. While quite a few counties (and possibly baronies) are part of the same demesne, there are still thousands of rulers on the map at any given moment. Each ruler has their own court, which has a minimum number of adult courtiers (I think it is six, but I might be wrong) below which new courtiers are generated (and an additional restriction is that there should always be an unmarried woman below a certain age, so old women don't count). That gives you 5-10k courtiers alive at any point. Even getting rid of prisoners, children, incapables, imbeciles, and others that might not be expected to do much, you still have thousands of courtiers running around. To have something to do while sitting around as a courtier, you'd probably want there to be a few events per year per courtier rather than a random chance that some courtier in some court gets an event. With an average of two events per character per year and a lower estimate of 5k courtiers you arrive at roughly 27.4 additional events firing every game day; and that's only if these two events don't in turn give rise to new events and if we use a very small court size. Some of these events might be slightly expanded versions of events that already fire for courtiers, but to have the events be fun for a player you'd probably need to add more stuff.
Assuming courtiers were to actually be able to keep their dynasty going rather than dying out if their liege doesn't take pity on them, you'd need to have large courts, which would mean increasing the court size and the threshold at which courtiers start to have less children. This would also lead to more living characters, which would lead to more event spam, which would lead to more slowdown. Unless you also want the events to affect the liege (read: cause yet more event spam) these events would probably amount to little but slowing down the game (unless you want to make playing behind the scenes as a courtier extremely powerful, in which case CK2 would no longer be CK2 as courtiers in general don't impact things to a great extent), which isn't something that is wanted as people already notice performance issues to various extents.
Aside from event spam you'd run into the problem of having no power. Your biggest tool for going against your host would be to start a plot (with him being able to imprison you fairly easy). With this insignificant power, you can't really expect to be able to refuse your liege when he starts making decisions about your family, marrying your daughter off to your rival, sending your son to church, and randomly imprisoning your wife because the AI wondered what the magic button did. It might not be the literal end of the world, but you'd probably want to go off on an adventure as quickly as possible (and hopefully before your liege tells you to go to a holy order, gives you a barony, or otherwise ruins everything). At that point, you would either win the war (and thus end up landed, making further courtier play likely to be unnecessary in that campaign and thus wasting a lot of effort on gameplay that barely is used), die (and perhaps play as your heir, if you have one (who might not be married (and who would be unlikely to get married as lieges generally don't marry all of their courtiers to someone), might be married in the wrong fashion, or might otherwise be in a terrible position), or end up imprisoned (which could lead to all kinds of unpleasantries and possibly lead to spending a few decades staring at a wall unless prisoners also get more events, which leads to more event spam).
There is one possible way to get around a lot of the event spam, which would be to have the events only fire for a human player. Personally, I don't like it when the player gets special treatment (positive or negative) or the AI otherwise is unable to use a feature at least somewhat competently, but leaving aside that you immediately run into a big problem: The time/effort needed to make unlanded characters playable (and to make them fun) would be quite large for the amount of use that you get out of it (as you likely will end up landed at some point, willingly or otherwise), making it unlikely that the devs would be willing to spend time on it.
TL;DR: The slowdown would be terrible and the addition of being able to play a courtier wouldn't really add that much. Just use the console/mod the game to land your ancestor and play as him normally, or trigger the adventurer event for him with the console and take over if he wins.
