That still can all be calculated and designed if you know what you are doing. That is not an excuse. Just make a study of how MySQL optimizer make it's choices, then see what indexes are available, what the distribution of values is among the columns that you are using for join/where/order, the number of rows in each table, the average length of the data in these columns, etc.. You can predict pretty good if somehting is optimized or not.
I personally don't use p-connect, but some people swear by them. Again, depends on the how busy your site is, and your hardware. With enough hardware, p-connect will speed up your site a bit. But if you want to limit connections, switch it off.