I have not personally seen the fatal hit error and any possible fix or resolution for it is uncertain so I too would be careful. The best results I have seen people talk about (in order fix it) claimed in this thread but be careful, nonetheless.
That was the thread I used (after some desperate searching!) and I was pleased and slightly amazed when it worked....however I think we agree it does not always work and it maybe that my NAND is getting dodgy, so repartitioning for data on ext....a worry....so not yet.
As for the lag - kep in mind that software (apps) is getting bigger and more demanding of hardware and memory while the HD2 is somewhat dated and limited with the 400 or so MB of RAM. Splitting the load by using DataOnEXT can help. More aggressive min free settings, changes to your card's FAT32 sector size and read buffer can also make a difference.
If you cannot try DataOnEXT I would certainly consider a Class 10 card with an EXT4 partition and aggressive min free settings. Also, make sure you have as few apps loading / starting up as you must have since they all take memory from other things.
I have also used a class 10 card but that seemed that same - so I went to class 6, as I could not find a class 10 that was not UHS1 which was not recommended (the class 10 I used was obviously also UHS1)
I've now used setminfree app and set to "aggressive", I've also set max background to "no more than 2". I still feel I'm stumbing around randomly, I've looked for but not found any "guide" to tuning to put some structure on this and know when I am at "the best it can be".....I want to see how good I can get it as frankly at the moment I suspect the best user experience is to go back to Gingerbread which upsets me as an official geek