Hey, sorry for the late reply here.
Usually the client down slows down because:
Your drive is close to being full and/or very fragged
Your drive is really old, slow, or encrypted and running through an old CPU
Your drive is being used by another HDD-heavy process (like another active hydrus client or a defragger, lol)
You have loads of files in your inbox
I wrote some other bad code that is running specifically slow on your machine.
I fixed the large-inbox-slows-result-construction bug this week, so if that is true, please try again with v154. Also, I have added a comprehensive db-profiling mode in v154, which will record how long any kind of database access takes and dump lots of related information into the log. It would be very useful for me if you ran some profiles on your slow computer next week and sent me those results, as I can figure out exactly what is taking so long and clean it up. I will explain more about this new profiler in my v154 release post.
On your laptop, do those memory allocation errors cause the program to crash completely, or do you get a popup inside the client?