Windows file sharing over l2tp ipsec vpn

Question

I'm trying to connect remotely to a local pc to access its files. 
 Remote connection to utm vpn server establishes successfully. I can see the shared folders of the pc behind the UTM. 
 However, when I try to copy a remote file (behind the utm) to the vpn client machine it'll start to transfer but then stall out. Bandwidth is not an issue. UTM is connected to gigabit capable internet, and vpn client is comcast 200+ mbps capable. Both of these 'pipes' likely exceed the cpu encryption capabilities of the pc running utm (i5 5250u). 
 Here's the firewall rule I established which I believe should allow full traversal in both directions. 
 
 Internal network is the local lan behind the utm (10.10.1.0/24) 
 jsolo (usernetwork) is the ip (10.242.3.2 typically) assigned to the l2tp/ipsec user account when logging into this vpn. 
 I'm thinking perhaps instead of referencing the user account, the entire 10.242.3.0/24 network should be allowed? 
 Perhaps I'm overlooking some other requirements for file sharing? Maybe l2tp w/ ipsec is not the best vpn service for this? 
 I did have to disable the firewall on the pc behind the utm to even be able to ping it. Once I get the utm settings figured out, i'll add the 10.242.3.0/24 as an allowed remote network. 
 Thoughts, suggestions please. I'd really like to get this working. 
 Thanks!

Jay Jay · Accepted Answer

Finally some good news to report. The cable connection I used for testing had a dl/ul speed of 175/25 mbps respectively. I need a faster connection to find my utm box's actual limits given gigabit internet speeds in both directions. Establishing the l2tp/ipsec tunnel on the local lan to the utm (a sort of loopback mode?) achieves speeds around 300-350 mbps in either direction with ips off and about 250 mbps in either direction with ips on. These numbers are from speedtest.net and dslreports speed test. 
 Setting the isp gateway, utm WAN and LAN MTU all to 1472 allowed for full bandwidth of the above cable connection when connecting over L2TP/ipsec even with IPS enabled (albeit utm cpu usage was high). Files copied over from remote windows shares at ~20 MB/s which equates to ~160mbps. Figuring overhead this reasonably good speed. This correlates given the above max local speeds achieved (250-350 mbps). 
 I played around with setting the above 3 MTU's at various combinations of 1472 and 1500 bytes. It seems setting any of those to 1500 would induce slower throughput so I left them all at 1472. I suppose this makes some sense. If any are at the higher value then fragmentation results. 
 Performance with SSL vpn (openvpn) was much slower, ~40 mbps. I think it's an mtu issue also but don't see any place to adjust it in the UTM or custom settings. Changes can be made to the client config file so i'll try making some tweaks there.