I don’t have any specific recommendations. Every site’s anti-spam needs are different, what works for me may be unusable for you. There are all sorts of plugins available that are free or at least free to try. Evaluate a few for a while. Weigh speed against the mis-identification rate on your site. I think it’s worth tolerating a little bit of spam in order to gain speed.
On further thought, the Network Developer tool I mentioned probably does not have enough granularity to identify a specific plugin as the bottleneck. All the same, it’s a useful tool for identifying other page speed issues. It’s thus worth learning about even if it does not help in this instance.
Rather OT at this point, but FYI:
All major browsers have a set of developer tools, some of which are useful to non-developers. Ctrl-shift-I will typically open the UI. Otherwise look for something in the browser menu regarding tools. The resulting UI will have several different tools listed across the top. Go to the Network tool. Reload the page, submit a form, or navigate to start logging HTTP requests.
It’ll be readily apparent which requests take a relatively long time. You can click any particular request to see more information like headers sent and received and any data sent.
While you have the developer tools open, check out the Elements tool, where you can evaluate and alter/test the applied CSS for any page element. It has a mobile device emulator that is very useful in responsive theme development. The Console tool is mainly a JavaScript error log, something to check if a page is behaving strangely. The other tools have their uses, but are of lesser interest to most users. Don’t be afraid to experiment with these tools, you cannot break anything that a reload does not fix.
