Slow Gigabit ethernet speeds on 2013 MacPro

I dont know if there is some hidden setting im missing

There are *many* hidden settings on ethernet networks that could be affecting throughput rate. Ethernet can be tuned for many different models - the needs on a low-latency LAN connection, for example, may be very different from those for a high-latency trans-oceanic. Apple tends to default their systems to a generic, works-well-for-most setup, which may not be optimal in specific situations.

Unfortunately knowing all the options, their meanings and potential impact can be a life-long journey for many people. Things like buffers, maximum segment sizes, window scaling, slow start, flow control, checksumming and selective acks are good candidates.

Off hand I have no idea how to extract these settings from your Windows host (but I assume Google is your friend here). On MacOS, you can use a combination of netstat and sysctl to tune your network settings (the good thing is that these changes are generally transient, so if you screw up, a simple reboot fixes it.

Start with sysctl -a | grep net.inet for a list of tunable parameters.

Beyond the network, though, there may be differences in the networking protocols in use. I don't know enough about the Synology to know what protocol(s) it supports, but if the PC is running CIFS/SMB and the Mac is using NFS or AFP, that could also be a factor. Make sure you're testing similar protocols on both sides.

Lastly, for now, on a point-to-point connection link aggregation will do nothing for you. Link aggregation pseudo-randomly balances sessions over multiple connections but any session between two specific devices (e.g. between your Mac and your NAS) will only traverse a single link (to avoid out-of-order issues at the receive side). Link aggregation is useful if you're running multiple concurrent sessions (e.g. a server responding to multiple diverse clients) since the load will be shared, but every connection is limited to the maximum speed of the individual links in the aggregate, not the sum of all links. If you had two NAS boxes and a 2gpbs aggregate, you might be able to read/write to them at 1gpbs each, or you might be able to download from the internet at full speed over one link while simultaneously writing to the NAS over the other, but a single sessions will never exceed 1gpbs.

There are a few brute force things you can check for before diving into the rabbit hole of performance parameters.

Gigabit Ethernet requires 8-conductor cables. If you set the Mac for manual Gigabit speed, an inadequate cable will NOT allow you to make a connection at all.

There are generally not enough frame numbers to run 1500-byte frames at full speed, so you need to increase the frame size. It is generally safe to increase it to the 9000 byte limit, but some equipment would run slightly faster at about 80 bytes under 8192 = 8K size.

Most Routers, including Airport base stations, can not handle frames over standard 1500 size -- you may need a dedicated Gigabit Ethernet Switch that supports large frames. They are not expensive.

Camelot already covered the fact that Link Aggregation provides no benefit for a single connection like this one, only for a complex composite stream with multiple sub-streams that can be split across the two paths.