Your speed tests are low, they should be in the upper 80% to 90% range. Full page cache causes all sorts of other problems primarily with stale content and the fact that it doesn't help in the checkout process which is where you lose 60-80% of customers. Some of the largest stores are 400ms ttfb, it's very difficult to archieve 150-200ms without fpc so for most the realistic range is 300-500ms. The fpc is reliant on business circumstances, the most tested is Varnish but it needs to be a careful decision due to the tradeoffs, our environment doesn't use fpc but it's based around realtime pricing so needed the performance without it, we're setup with Redis. If your performance is still a problem due to code your next move is cluster hosting.