Knowing your general opposition to going through an upgrade if at all avoidable (though 4.7 to 5.2 is quite a smooth and generally painless upgrade), one thing that can have immediate benefits is to use block cache. It creates cached versions of all your blocks and makes the cached versions available in the blocks administration. Then you can replace your existing blocks with cached versions, as appropriate. Each block has its own configuration settings to determine how frequently it should be refreshed.

This will not affect node loads, menu loads (unless you use cached versions), comment loads or global user permission checks, etc., but it can help sites on underpowered servers. Figure 20-30 minutes of configuration time because you need to configure each block separately, and the cached versions will not automatically inherit the block visibility settings you may have on each block. You do not need to take your site down to do this, either.

Hope this helps!

Laura