Speed Limitation

If the ISP's gateway/router device has not been power cycled since the speed increase, I would do that first. 

On the CLI perform diagnose hardware deviceinfo nic <inteface name>

Output should look similar to this:

# diagnose hardware deviceinfo nic wan1 Description         Intel(R) Gigabit Ethernet Network Driver Driver_Name         igb Driver_Version      5.0.6 PCI_Vendor          8086 PCI_Device_ID       1533 PCI_Subsystem_Vendor          ffff PCI_Revision_ID     0003 PCI_Bus             5 PCI_Slot            0 MAC_Type            6 PCI_Bus_Type        PCI-E PCI_Bus_Speed       2.5Gb/s PCI_Bus_Width       Width x1 IRQ                 16 System_Device_Name  wan1 Current_HWaddr      90:6c:ac:3e:cc:xx Permanent_HWaddr    90:6c:ac:3e:dd:cc Link                up Speed               1000 Duplex              full FlowControl         current:0/requested:3 Interrupt mode      MSI-X Rx queue(s)         1 Tx queue(s)         1 Rx_Packets          186009705 Tx_Packets          142789065 Rx_Bytes            221319840722 Tx_Bytes            21885484538 Rx_Errors           0 Tx_Errors           0 Rx_Dropped          0 Tx_Dropped          0 Multicast           2130115 Collisions          0 Rx_Length_Errors    0 Rx_Over_Errors      0 Rx_CRC_Errors       0 Rx_Frame_Errors     0 Rx_FIFO_Errors      0 Rx_Missed_Errors    0 Tx_Aborted_Errors   0 Tx_Carrier_Errors   0 Tx_FIFO_Errors      0 Tx_Heartbeat_Errors 0 Tx_Window_Errors    0 Tx_Single_Collision_Frames    0 Tx_Multiple_Collision_Frames  0 Tx_Deferred         0 Rx_Frame_Too_Longs  0 Rx_Frame_Too_Shorts 0 Rx_Align_Errors     0 Rx_Flow_Control_XON 0 Rx_Flow_Control_XOFF          0 Tx_Flow_Control_XON 0 Tx_Flow_Control_XOFF          0 Rx_Control_Unknown_Opcodes    0 PHY_Media_Type      1 max_frame_size      1522 CTRL                00100241 STATUS              00280783 TXCW                00000000 RXCW                00000000 avd/ctrl            0de1/0200 Look at the Link speed and duplex lines and check the various rx/tx error/dropped counts.  Going to assume the D200 is connected to the ISP's gateway/router device at either 100 or 1000 full duplex - if you see half duplex then I would say there's a problem.  If there are rx/tx error/drops counts, wait while then perform the diag test again to see if the counters increase.

If there are no error check the interface configuration, eg.

config system interface edit "wan1" show

and look for any speed, inbandwidth, and outbandwidth lines.

Further troubleshooting may including direct connecting a laptop to the ISP gateway/router and running speed tests, contacting the ISP to confirm their equipment is properly configured, suspecting a possible network switch issue.