@Torm: Thanks, glad you enjoyed it. Also thanks for the beastiary and your mods which helped me learn decorate better and gave me some ideas for the mod.
@ Nova & David: Thanks again for the feedback. My PC is 5 years old, but it is a P4, 3Ghz with a 6800GT video card, and I have never encountered the slowdown you mentioned, which probably is not surprising, considering the specs of my machine. If you don't mind, I have some things you can try to help me pin this down.
Firstly, there is a line in the <yourusername>-zdoom.ini file which has 'scoredoomstats=true', try playing with this set to false (you can do this via the console as well), and see if you still get the lag. This will not display the scoredoom HUD during gameplay, though the scoring bonuses on the upper L.H.S. of the screen will remain. I have calculations in the HUD code, which gets called every tic, which is not the most efficient way of doing it, and could possibly be contributing to the lag.
Secondly, looking back, I gather this lag is more apparent after playing 5 levels or more continuously. If you save the game after it happens, and restart scoredoom completely and load at the same point, does it still keep happening? (If not, then this could be a possible workaround, until/if a proper fix comes out

)
Thirdly, the version of gzdoom it was built on (1.0.29) I think was fine, but I decided to upload an older version of gzdoom which was built on gzdoom 1.0.27(?) for you to try out to see if you still get the lag. You can grab it here:
http://files.filefront.com/gzscoredoom1 ... einfo.html
If you don't get the lag for that version of scoredoom, its still possible that it's something in my code I added since that version, though I cant think what.
Despite the extra calculations in the scoredoom HUD code, the damage code has not change significantly at all from gzdoom's damage code, except for a few more conditionals and writing damage scoring info to the console/HUD similar to pickup messages every time damage is given out. Nothing there which looks out of the ordinary, except the super shotgun blast full-on, does generate at least 20 console damage messages in one game tic! Perhaps its something to do with how all this is affecting zdoom's console buffer code for lower spec PCs? (I'm thinking this is *probably* it)
Looking at how I'm handling these messages vs say how pickup messages are handled, I have the damage messages set to a higher priority than pickup messages (not necessary). Though I don't think that's important, since if I did set the damage messages to lower priority, all 20 damage messages will all still be the same priority relative to each other and it looks like zdoom's console code will behave exactly the same, but I suppose its worth a try to see if lowering the priority will help performance.
Anyways, let me know what you found from the above, as this will help me decide what to do for a fix, should I get time to release one. I'm hoping it will be something minor. Thanks.