Xfinity Superfast still slow

I recently upgraded my Internet connection plan with Xfinity from Internet 200 Mbps plus TV to 900 Mbps Superfast plan (with TV plan dropped.  Who cares with TV these days?).  It was a good deal plan, as I now pay less with a lot faster nominal Internet speed.

In the beginning, after waiting an hour as told by the customer service, I saw the upstream speed improved to 20+ Mbps, but the upstream speed did not increase (stayed at around 140 Mbps).  A few calls/chats with the customer service representatives (they sent a few update signals) and modem and router reboots nothing improved.

After the last call to them, they decided to send a technician to visit the next day.  I had tried almost everything (except setting the router's settings to factory defaults). My home network setup is little bit complicated, with the AP router and DOCSIS 3.1 modem sitting in a mounted rack in the walking closet.  

The coaxial cable from ISP is split through a 1-to-8 Moca-Ready splitter (5-2300 MHz, -11 dB drop between Out and In), because I need to connect the TV in the living room through MoCA as well as some bedrooms and home office.  In the beginning, I thought the chocking was due to interference with MoCA (as the DOCSIS cable modem shared the same coax medium with MoCA modems).   Not sure whether I still need to change that splitter (-11 dB drop is a little too much, not counting the bandwidth is only up to 2300, not 2400 or 2500 MHz.  But when I checked the MoCA specs, the frequency span for MoCA 2.5 is from 1002 to 1675 MHz.)

Out of my patience (I use the Internet on a daily basis as part of Work From Home with Citrix connection, hence requires constant and speedy internet link), I decided to simplify the connection by removing the set-top box (which soon I'd return as I'd no longer have TV service), so the cable modem was wired directly to the ISP (not through splitter).  This did not resolve the data rate, although it improved the SNR in the modem.

Out of my frustration, I factory reset my Synology Router Rt2600a and redid the speed test.  And...voila! I got 700+ Mbps.  Yes, it's still far from 900 Mbps as promised, but at least it's 4x faster than what I got before.  Turned out, the "Threat Prevention" add-on feature in Synology Router was the culprit.  It was CPU-intensive processing, which shouldn't be performed by a normal CPU, probably by a special or dedicated CPU doing this kind of inspection and prevention.

I still really need to have an intrusion prevention feature as it has been securing my home network heavily from hackers and spam.  Perhaps it is time to shop for a dedicated intrusion prevention device.