Nice looking bridge, just a few things are bothering me.
The first thing I noticed is the handrail is one solid piece. Where I’m not sure that’s impossible, from looking for reference images I noticed most stone bridges are made of many pieces. Perhaps you should divide the support rails from the bridge and the top of the rail from the support rails. Just putting a little crease between those parts will make them look like they were made separately and then assembled. I would also seperate the handrail and the line of stone the support rails are on en between every rail (or every other) again to make it appear assembled out of pieces. The only way a bridge like that could be constructed out of one solid piece would have required there be a giant stone blocking the stream and they carved it down, which probably wouldn’t be possible, but if it were the whole bridge would be one solid piece, including the footpath.
About your grass particles, are you using child particles? you should be able to get away with quite a large number of children particles without an issue. If you are then perhaps render the bridge and the bg separately? maybe even separate the bg into 2 layers if there is still an issue.
Apart from that I really like the texture on the walkway and the detail in the hand rail. Maybe make your lighting a little bit stronger but apart from that nice job!