Basically, anything that is not an inert block can cause a bit of lag.
RP's micro blocks are a bit bad in that aspect in that they each count as a "tile" entity, requiring their specific set of render and actions for each tick calculation.
As they are also set as "transparent", they allow light through, and allow for more blocks to be updated by light level updates.
As for the relays, they are just a bit worse than a hopper, but I am not sure how it compares to 2 hoppers on top of each others, that would give nearly the same amount of buffer.
Bottom line is, if you don't need a big buffer, a hopper should be enough, but if you do need large buffer, choose whatever solution fits your design better, as hoppers can only go vertically where relays can go any direction.
But it's really all about not putting millions of the same things, really.
Back in the days where advanced/compact solar were not widely used (or even existent), solar farms of hundreds of solar panels were causing massive lag for servers. So does spamming geothermal generators, watermills, windmills, normal generators, and even the large array of engines for MJ power types.
Oh, and low count timers can also quickly degrade the fps and tickrate.
