Will plugging the Samsung fast charger into a outlet rated 10W/2.1AMP slow down the charge rate? The S6 comes with a fast wall charger, but how does it work exactly? 

I want to plug it into a “one outlet Belkin” with surge protection, that is 2.1AMP for the ome USB port and 10W for the single AC outlet. 

Now my question is will plugging the charger into the Belkin slow down the fast charge time?
First part of your question How do fast Chargers work

Firstly one needs to understand C-rate or charge rate. Quoting from the linked article


  C-rate is a measure that governs at what current a battery is charged and discharged. At 1C, a battery rated 1,000mAh charges at a current of 1,000mAh. In an ideal world the battery would be fully charged in 60 minutes. At 1C, the same battery discharges at 1,000mA.


Normal Charging


Li-Ion or Li-PO (both have same charge/discharge characteristics) -typically charged between 0.5 C to 0.8 C, more often closer to 0.8 C being the recommended rate. Quoting from BU-401a: Fast and Ultra-fast Chargers



  The longevity (of Li-Ion batteries), can further be prolonged by charging and discharging bellow 1C; 0.8C is the recommended rate.



Li-Ion batteries go through three stages of charging of battery charging Figure 1 of Charging Li-Ion batteries- Constant Current, Saturation Charge (current decreases), Ready (almost zero current). If you plot current drawn using an app like Ampere while charging a battery from 10% or lower to 100%, you would observe similar graph (you will not see the first stage unless fully drained and measured by proper electrical equipment)


Fast Charging


Essential difference is the C-rate or Charging rate. These chargers typically work by charging at 1 to 3 C rate
When you charge by such chargers, the second stage of charging (saturation stage ) is skipped and the battery reaches the third stage (ready stage) without being fully charged Quoting from this



  .... chargers may use the simplified “charge-and-run” method that charges a lithium-ion battery in one hour or less without going to the Stage 2 saturation charge. “Ready” appears when the battery reaches the voltage threshold at Stage 1. State-of-charge (SoC) at this point is about 85 percent, a level that may be sufficient for many users



Is it good for the Battery? No. Figure 2 here graphically shows deterioration in long term battery life measured in Cycles and the graph shows cycles for 3C charging dropping to 400 cycles compared to 500 for 1C ( I have no idea at what C, Your charger works. I Couldn't
locate any OEM information)


Second part of your question Will plugging the charger into the Belkin slow down the fast charge time?


Going by the specs provided by you: Probably No (linking your Belkin model would help as there are many variants going by your description)
USB rating is same as that of standard USB and power rating of 10W doesn't make a difference (chargers will use as much power as required and not more just because outlet is capable of providing more)
You can compare the charging times reported So, We Fast Charged the Samsung Galaxy S6 and This is What Happened here and update your findings in your question to help others