Memory leak means, some parts of the software take up some memory and don't release it anymore when they are finished. This causes more and more memory to be allocated, causing the graphics adapter to run out of memory. This causes massive lag that only can be solved by closing the whole software (i.e. the Enhanced Client).
It is not normal for a game to run slower over time, if it is programmed correctly. In the case of the EC, the client literally breaks down within 10 minutes if certain particle effects occur on your screen. If you have a graphics adapter with more memory, it will probably take longer to crash.
I use a 512 MB graphics adapter. While I don't have any noteworthy lag in game, usually I had to restart the client 3 or 4 times during a Slasher fight. Since the latest patch, I don't have to restart anymore, but I still notice abnormal increasing lag during this battle. I now have to restart the client only 1 time (at the end of the battle). Thus, the problem seems to be less severe now, but it still is there.
The lag you experience when moving the mouse over the black part of the screen (I could not reproduce that) is most likely a different issue.
First you load things you need into the memory, but deleting it instalty may cause many problems, think of rejoining the same area, you have to load all textures again from the harddrive, this is slow. In Half Life for example you have a good system of level loading for this, when you enter a new area old stuff will be removed and new stuff is loaded in, this needs time, but is no problem for the half life engine because there are loading screens. In UO it isnt possible so easy, because you have no loading screens or teleport loading screens, so you have todo it at the same time and this costs power looking for what to delete, is like looking into a table at each entry and decide if it should be removed or not and if even one entry is deleted wrong it happend what happend in KR many times it crashes.
I dont want to negate your problem, maybe there is a leak in the animation system but not for me! I have done many slashers and only for one i can report a bug, the client slows down at 3 fps and stucks there a relogin corrects it.
But it can also be possible that it is a hardware specific problem, maybe that such a cleaning function is not working correctly for your machine, too fast for example or too slow and then it happens that you got such strange performance lost.
Lets compare the hardware, i have:
Intel Q6600
4 GB Ram
ATI HD 4890 - 1024 MB
1 TB Harddiskspace (RAID 0, no SSD)
ASUS P5Q Mobo
Windows 7 x64
Have you tried to reduce or raise the cache in the settings? Again i want to help not to critize you!